What is a pigment doing in a wash?
Tiny coloured particles are dispersed through the paint. A binder holds them together and helps them stick to the paper as the water evaporates. In traditional watercolour, the binder is usually gum arabic. Add more water to your brush and the same blue produces a paler wash.
What is a pigment code on a paint tube?
Look for a combination of letters and numbers. In the Colour Index, PB identifies the blue family and the number identifies a material within that family. Winsor & Newton’s Professional Watercolour in French Ultramarine gives a good example. Its code is PB29. The product sheet also lists 263, which is the number the manufacturer assigns to this colour in its own range.
Compare two tubes in the shop
Two makers might both call a blue ‘ultramarine’. Check the codes when you compare them. A single code means the maker lists one colouring material. Two or more codes indicate a mixture. Even two paints marked PB29 can handle slightly differently. If you find a blue you want to buy again, keep a record of its brand, colour name and code.
Make a swatch you can repeat
Draw a line in waterproof ink on a scrap of the paper you normally paint on. Let the ink dry, then brush a band of blue over the line. Rinse your brush and draw out one edge of the wash with clean water. You now have a concentrated patch and a diluted one on the same card. Lay it flat to dry and write the tube details on the back.
Look at the dry wash
Check how clearly the ink line shows through and where the blue settles in the hollows of the paper. The manufacturer describes this particular ultramarine as transparent and granulating. A finely mottled finish can be part of its character. To compare another blue, use the same paper and a brush of a similar size.
Slip your swatches into an envelope in your sketchbook. Next time you need a colour for a shadow or a wash, you can pick it from a sample painted on your own paper.
Use the swatches to pick two colours for a leaf print on a piece of card.
Winsor & Newton’s French Ultramarine product sheet